Such fluid working machines are generally used, when fluids are to be pumped. Such pumping of fluids can relate to both gases and liquids. Of course, the word fluid can even relate to a mixture of gas and liquid and furthermore to a supercritical fluid, where no distinction between gas and liquid can be made anymore.
In particular, such fluid working machines are used, if the pressure level of a fluid has to be increased. For instance, such a fluid working machine could be an air compressor or a hydraulic pump. Fluid working machines generally comprise one or more working chambers of a cyclically changing volume. For each cyclically changing volume, there is provided a fluid inlet valve and a fluid outlet valve. When the volume of the working chamber increases, the fluid inlet valve opens, while the fluid outlet valve closes. Therefore, fluid at the low pressure level is sucked into the working chamber. As soon as the volume of the working chamber has reached its maximum and starts to decrease, the fluid inlet valve closes. When the fluid pressure within the working chamber has reached the pressure level of the high pressure level, the fluid outlet valve opens and fluid is ejected to the high pressure side at the high pressure level. When finally the working chamber has reached its minimal volume, the fluid outlet valve closes, the volume of the working chamber starts to increase again and the fluid inlet valve opens. Therefore, the pumping cycle starts again.
Traditionally, the fluid inlet and fluid outlet valves are passive valves. I.e., the valves open themselves under the influence of a pressure difference on both sides of the valve. Of course, the valves open in only one direction, whereas in the closing direction of the valves, the valves are closed, independent of the magnitude of pressure difference. A typical design of such a valve is a check valve or a poppet valve.
Lately, there were proposed synthetically commutated hydraulic pumps, where the opening and closing of the inlet valves and/or the outlet valves is controlled by a controlling unit of the synthetically commutated hydraulic pump. Those synthetically commutated hydraulic pumps are also known as digital displacement pumps or variable displacement pumps.
The advantage of such a controlled opening and closing of the inlet and/or outlet valves is, that several modes of operation of the hydraulic pump can be achieved. If the inlet and outlet valves are controlled in a way, analogously to the traditional passive opening and closing of the valves, a full stroke pumping mode is achieved.
However, by appropriate control of the opening/closing state of the valves, different modes can be achieved. For instance, if the inlet valve is held open during the whole working cycle of the pump, a non-pumping mode can be achieved. In this mode, the fluid is sucked from the low pressure fluid reservoir and pushed back into the low pressure fluid reservoir, during a full cycle of the working chamber. However, no effective pumping to the high pressure side of the pump is performed.
Another mode can be reached, if the fluid inlet valve is held open during part of the volume decreasing stroke of the working chamber. If the fluid inlet valve is closed after the working chamber has reduced to e.g. half of its size, only half of the available pumping volume is used for pumping. The pumping flow rate is therefore at approximately 50% of the maximum. Therefore, a partial stroke pumping mode can be achieved.
Said three pumping modes can be realised, even if only the fluid inlet valve is actively controlled, and the fluid outlet valve is still of a passive type.
However, even more modes of operation can be achieved, if the fluid outlet valve can be actively controlled as well:
The fluid outlet valve is held open during the phase, where the volume of the fluid working chamber increases, while the fluid inlet valve is held closed at the same time. The state of the fluid inlet valve and the state of the fluid outlet valve are interchanged, when the volume of the working chamber decreases. This way, fluid can be transferred in the reverse direction, i.e., fluid is transferred from the higher pressure level to the lower pressure level. The energy stored by the elevated pressure level of the hydraulic fluid can be used to drive the fluid working machine. Therefore, the synthetically commutated hydraulic pump is used in a motoring mode.
By controlling the opening/closing state of the inlet valve and the outlet valve, in the sense of the partial stroke pumping mode, a partial stroke reverse pumping mode a partial stroke motoring mode can be achieved as well.
Independent of the actual design of the valves as actively controllable valves or as passive valves, the inlet valves and the outlet valves are usually placed in a fluid inlet channel and a fluid outlet channel, provided in the fluid working machine, respectively. The fluid inlet channel connects the low pressure fluid reservoir with the working chamber of the fluid working machine, whereas the fluid outlet channel connects the working chamber of the fluid working machine with the high pressure side of the system. According to the state of the art, the fluid inlet valves and the fluid outlet valves are connected to the fluid working machine through separate access ports. For example, two fluid channels are provided in the cylinder head portion of a fluid working machine. A fluid inlet valve unit is assembled to the fluid working machine's body by inserting it from the outside into the fluid inlet channel. Likewise, the fluid outlet valve is assembled to the fluid working machine by inserting it from the outside into the fluid outlet channel. Therefore, two access ports for mounting the two valves are used.
Such a design, where each cylinder of the fluid working machine needs at least two separate access ports causes problems. The access ports and the corresponding cavities have to be machined into the pump body. Therefore, a high number of machining processes has to be performed. If the pump body is molded, a relatively complex molding process has to be performed. Furthermore, the machining of the raw parts is relatively complex.
Additionally, because of the increased number of components, which have to be installed in the pump body, the number of assembly steps is high. Furthermore, the amount of sealing points is relatively high, as well. The latter one increases the possibility of fluid leakage as well.
Both U.S. Pat. No. 5,190,446 and US 2006-0039795 A1 show examples of synthetically commutated hydraulic pumps, according to the state of the art. The fluid inlet valves and the fluid outlet valves of the working chambers of cyclically changing volume connect to said working chambers through separate access ports. In the embodiments shown, the direction of the fluid inlet valve is arranged perpendicular to the direction of the fluid outlet valve. The valves are assembled to the pump bodies through separate assembly ports and are installed in separate cavities.